I have analyzed a type of concept, the collection, which is not readily characterized by the traditional class model. Collections (e.g., forest, army) differ from classes (e.g., trees, soldiers) in their part-whole relationships, the internal structure and the nature of the higher order units they form. The simple relabeling of identical elements as a collection (e.g., forest) or a class (e.g., trees) changes the nature of the organization subjects impose on an array and consequently results in differences in cognitive functioning. This past year, I have found that the aggregate structure of collections help children solve number problems. Number is relevant in that it is a characteristic of sets of objects and not of the objects themselves. Collection labels helped children to conserve number, to access and verbalize a numerically relevant basis for their judgments of equality and difference, to judge equivalence based on number rather than length, and to use the cardinality principle. Another issue studied was concept acquisition. when children learn hierarchically organized concepts they have to learn the relation between the two terms applied to the same object, e.g., "poodle" and "dog." The present analysis suggests that when learning a new set of concepts, children would be likely to impose a collection like organization on the hierarchy unless there is enough information forcing a class inclusion organization. When novel class inclusion hierarchies were taught to children only by ostensive definition (pointing and labeling), children erroneously imposed a collection-like structure on the hierarchy.